The British father of Oregon gunman Chris Harper-Mercer says his son's brutal killing spree would never have happened if he hadn't been able to compile a huge arsenal of guns.

In an interview which is sure to inflame the debate about America's gun laws, Ian Mercer says the only reasons people want the weapons are to kill.

The 55-year-old, who lives in California, stopped short of blaming the availability of guns for the killing of nine people at an Oregon community college, but he is adamant things must change in the country he now calls home.

"How on earth could he compile 13 guns? How can that happen?

"They talk about gun laws and gun control every time something like this happens and nothing is done.

"I'm not trying to say that is to blame for what happened, but if Chris had not been able to get hold of 13 guns it wouldn't have happened."

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He added: "You don't see these kind of mass shootings all over the world on a consistent basis like you do in the United States so somebody has to ask the question, 'how is it so easy to get all these guns'.

"I've never held a gun in my life and never want to.

"I had no idea he had any guns whatsoever and I'm a great believer that you don't buy guns.

"The gun law has to change. How can it not?

"Even people that believe in the right to bear arms; what right do you have to take people's lives.

"That's what guns are. They're killers.

"It's as simple as that. It's black and white. What do you want a gun for?"

Student Anastasia Boylan, 18, made it out of Umpqua Community College alive after playing dead as the horror unfolded.

Her father Stacy told CNN victims had been asked to stand up if they were Christian.

He said: "And they would stand up and he said, "Good, because you're a Christian, you're going to see God in just about one second.

"Then he shot and killed them."

Police recovered four weapons at the scene, including three handguns and a long-range rifle, according to reports.

Harper-Mercer's preoccupation with firearms dated back at least to 2012 or 2013, when he sought to register for training at Seven 4 Para, a private self-defense and law enforcement academy in Torrance, California, where he lived at the time, said Eloy Way, president and head instructor for the centre.

"We wanted him to take a beginner safety course, and he was trying to tell me that he already had experience with firearms, and I didn't get a good feeling about him, so I turned him down," Way told Reuters.

"He was just kind of a weird guy and seemed kind of spoiled, immature," Way recalled. "He was a little bit too anxious to get high-level training, and there was no reason for it."

Way's concerns that Harper-Mercer might misuse the training he would receive at the academy proved prescient.